Solid state light sources may be utilized to provide colored (e.g., non-white) or white LED light (e.g., perceived as being white or near-white). White solid state emitters have been investigated as potential replacements for white incandescent lamps due to reasons including substantially increased efficiency and longevity. Longevity of solid state emitters is of particular benefit in environments where access is difficult and/or where change-out costs are extremely high.
A solid state lighting device may include, for example, at least one organic or inorganic light emitting diode (“LED”) or a laser. A solid state lighting device produces light (ultraviolet, visible, or infrared) by exciting electrons across the band gap between a conduction band and a valence band of a semiconductor active (light-emitting) layer, with the electron transition generating light at a wavelength that depends on the band gap. Thus, the color (wavelength) of the light emitted by a solid state emitter depends on the materials of the active layers thereof. Solid state light sources provide potential for very high efficiency relative to conventional incandescent or fluorescent sources, but solid state light sources present significant challenges in simultaneously achieving good efficacy, good color reproduction, and color stability (e.g., with respect to variations in operating temperature).
The term chromaticity is applied to identify the color of the light source regardless of the output intensity (e.g., lumens). When the chromaticity of different light sources is equal, the color of the light from each light source appears the same to the eye regardless of the intensity. The chromaticity of a light source may be represented by chromaticity coordinates. An example of such coordinates is embodied in the 1931 CIE 1931 chromaticity diagram, in which the color of the emitted light is represented by x and y coordinates. Color coordinates that lie on or near the black-body locus yield pleasing white light to a human observer. The 1931 CIE Diagram (FIG. 1) includes temperature listings along the blackbody locus (embodying a curved line emanating from the right corner).
Color temperature of a light source is the temperature of an ideal black-body radiator that radiates light of a comparable hue to that of the light source. An incandescent light bulb approximates an ideal black-body radiator; as such as bulb is heated and becomes incandescent, it first glows reddish, then yellowish, then white, and finally bluish (because wavelength associated with the peak radiation of the blackbody radiator becomes progressively shorter with increased temperature). Other light sources such as fluorescent lamps and LED lamps, emit light primarily by processes other than thermal radiation, such that the emitted radiation does not follow the form of a black-body spectrum. These sources are assigned a correlated color temperature (CCT), which is the color temperature of a black body radiator to which human color perception most closely matches the light from the lamp. The terms “color temperature” and “correlated color temperature” may be used interchangeably herein.
Because light that is perceived as white is necessarily a blend of light of two or more colors (or wavelengths), no single light emitting diode junction has been developed that can produce white light. White light production from solid state emitters requires multiple solid state emitters of different colors and/or some combination of at least one solid state emitter and at least one lumiphoric material (also known as a lumiphor, including for example, phosphors, scintillators, and lumiphoric inks).
Light perceived as white or near-white may be generated by a combination of red, green, and blue (“RGB”) solid state emitters (e.g., LEDs). Output color of such a device may be altered by separately adjusting supply of current to the red, green, and blue LEDs. Another method for generating white or near-white light is by using a blue LED and a lumiphor such as a yellow phosphor. In the latter case, a portion of the blue LED emissions pass through the yellow phosphor, while another portion of the blue LED emissions is downconverted to yellow, and the blue and yellow light in combination provide light that is perceived as white. Still another approach for producing white light is to stimulate phosphors or dyes of multiple colors with a violet or ultraviolet LED source.
When multiple solid state emitters and/or lumiphors are used in a single lighting device, the CCT and intensity (lumens) of the lighting device may depend on many factors, including (for example), operating temperature of the emitting components, age of the emitting components, and batch-to-batch variations in production of the emitting components.
A representative example of a white LED lamp includes a package of a blue LED chip (e.g., made of InGaN and/or GaN) combined with a lumiphoric material such as a phosphor (e.g., YAG:Ce) that absorbs at least a portion of the blue light (first peak wavelength) and re-emits yellow light (second peak wavelength), with the combined yellow and blue emissions providing light that is perceived as white or near-white in character. If the combined yellow and blue light is perceived as yellow or green, it can be referred to as ‘blue shifted yellow’ (“BSY”) light or ‘blue shifted green’ (“BSG”) light. Color temperatures over 5,000K are called cool colors (bluish white), while lower color temperatures (2,700-3,000 K) are called warm colors (yellowish white through red). When a BSY emitter is used, addition of red spectral output from a red solid state emitter (e.g., LED) or red lumiphoric material may increase the warmth of the aggregated light output. The integration of red LEDs into a blue LED BSY (“BSY+R”) lighting device improves color rendering and better approximates light produced by incandescent lamps.
When red supplemental LEDs are used in combination with high-power primary blue LEDs (e.g., as embodied in BSY components), it can be challenging to maintain aggregated emissions of such combination at a constant color point. Red LEDs include active regions typically formed of Group III phosphide (e.g., (Al,In,Ga)P) material, in contrast to blue LEDs, which include active regions typically are formed of Group III nitride materials (e.g., represented as (Al,In,Ga)N, including but not limited to GaN). Group III phosphide materials typically exhibit substantially less temperature stability than Group III nitride materials. Due to their chemistry, red LEDs lose a significant portion (e.g., 40-50%) of their efficacy when operating at 85° C. versus operating at a cold condition (i.e., room temperature or less). When red and blue LEDs are affixed to a common submount or in thermal communication with a common heatsink, heat emanating from the blue LEDs will increase the temperature of the red LEDs. To maintain a relatively constant color point utilizing a device including a Group III-nitride-based blue LED (e.g., as part of a BSY emitter) and Group III-phosphide based red LED, current to the Group III-phosphide based red LED emitter must be altered as temperature increases because of the different temperature responses of the blue LED and red LED. Adjustment of supply of current to different emitters responsive to a temperature signal is known as temperature compensation.
A representative LED lighting system in the art including arrays of red LEDs, an array of green LEDs, an array of blue LEDs, a single photodiode, and a temperature sensor, is disclosed in U.S. Pat. No. 6,441,558. The three arrays of LEDs are arranged in a light mixer arranged to receive power from a rectified power supply, with a controller being coupled to the power supply and light mixer. The controller includes optical feedback from a photodiode in combination with a feed-forward temperature compensation arrangement to maintain output at a desired color point and light output level by separately controlling supply of current to the red LED array, the green LED array, and the blue LED array arranged in parallel. Output color may be adjusted with a user input for color preference. U.S. Pat. No. 6,441,558 discloses use of a single photodiode for light sensing and a single temperature sensor for temperature sensing for the entire lighting device. In each array, the plurality of LEDs preferably has substantially similar electrical and optical characteristics. Chromaticity coordinates of the LED light sources are estimated based on the sensed temperature in combination with stored lumen output fractions as a function of junction temperature. Output of the light sensor and temperature sensor are used in combination with stored information to control each LED array to provide a desired light intensity and maintain a desired color point.
The LED lighting system according to U.S. Pat. No. 6,441,558 has various limitations that affect its utility. Use of optical feedback increases complexity and expense of the lighting device, and the optical sensor may restrict light output, increase device size, and/or affect aesthetics of the lighting device. Control of each LED array as a group does not accommodate possible variation in output characteristics for different emitters within a single array (as noted previously, output characteristics of LEDs differ due to natural batch-to-batch variations in production). Although variation in output characteristics between different LEDs of the same color to be used in a single lighting device may be reduced by sorting and binning (with selection of emitters have closely matched characteristics), such approach limits utilization of the full distribution of pre-manufactured LED components and therefore increases cost of the resulting lighting device. With each LED array arranged in parallel as disclosed by U.S. Pat. No. 6,441,558, at least six contacts (i.e., an anode and cathode for each of three LED color arrays) are required to supply power to the LEDs, thereby complicating wiring and fabrication of a resulting device.
Although U.S. Pat. No. 6,441,558 assumes that multiple LEDs have substantially similar electrical and optical characteristics, actual LEDs as produced by conventional manufacturing methods are subject to variation in such characteristics from batch to batch, thereby affecting their output intensity and output color. When multiple LEDs are distributed over a large area in a single light fixture and subject to control with the same control circuit, color point and/or intensity may vary significantly at different locations along the fixture. Moreover, temperature at various points of a light fixture may differ significantly, especially with respect to fixtures of large sizes (e.g., due to placement of heatsinks, proximity to external cooling or heating sources such as HVAC outlets or windows/doors, natural convection effects, etc.). Such temperature differences at different locations of LEDs within a single light fixture may lead to further variations in color point and/or intensity at different locations along the fixture.
Lighting devices including temperature protection circuits that terminate operation of emitters of the lighting device upon sensing of an excessive temperature condition are known. Such devices have limited utility, however, since an operator of such a lighting device may mistakenly assume that the device is defective when the device ceases operation upon detection of an excessive temperature condition. It would be beneficial to avoid misperception by lighting device operators of operational status of a lighting device when a lighting device detects an over-temperature condition.
Elongated lighting devices such as fluorescent tube-based light fixtures are widely employed in commercial and industrial buildings, as well as in some residential environments. Solid state lighting devices are capable of operating at much greater luminous efficiency and greater reliability than fluorescent tubes, but solid state lighting devices generally include small-area emitters that approximate point sources—in contrast to the large emissive area characteristic of fluorescent tubes. It would be desirable to provide solid state lighting devices similar in size and conformation to fluorescent tube-based devices to enable retrofit of solid state light bulbs or solid state light fixtures in the same or a comparable envelope of space.
It would be desirable to overcome one or more of the foregoing limitations associated with conventional solid state lighting devices.
This background information is provided to reveal information believed by Applicants to be of possible relevance to the present invention. No admission is necessarily intended, or should be construed, that any of the preceding information constitutes prior art impacting the patentable character of the subject matter claimed herein.